There is significant controversy surrounding the presence and nature of cognitive and language deficits in patients with Parkinson's disease (PD). The two-fold purpose of the proposed study is: 1) to detail performance on complex cognitive tasks such as sentence appreciation and naming by decomposing these into more basic components and evaluating each component using paper-and-pencil techniques; and to relate language deficits to impairments in other cognitive domains such as memory, attention, and speed of information processing; and 2) to relate cognitive performance to cerebral function with positron emission tomography (PET), where we will measure regional cerebral blood flow during the performance of specific cognitive tasks in control subjects and in patients with mild idiopathic PD. We hypothesize that typical PD patients will be compromised on specific linguistic processes such as the appreciation of sentence structure but not in their appreciation of semantics. This will be related to deficits in selective attention and reduced information processing speed, but not to a limited size short-term memory buffer. PET studies of cerebral blood flow during cognitive activation in normals are expected to reveal increased perfusion in a network of cerebral regions that subserves a complex skill such as sentence comprehension. Changes in the distribution of activation within the network will be seen depending on the particular linguistic or cognitive probe of the complex material. Mild PD patients are expected to reveal difficulty recruiting portions of cerebral cognitive networks, including dorsolateral and mesial frontal cortex and caudate, during attempts to process compromised material such as sentence grammar and attention.